Fix NPE when connect timeout

If user click connect button and then receive timeout event after leave
detail page, the NPE happened. Ignore the timeout event if activity
already gone.

Bug: 131141866
Test: manual test
Test: make RunSettingsRoboTests -j32 ROBOTEST_FILTER=com.android.settings.wifi.details.WifiDetailPreferenceControllerTest
Change-Id: Ic22c22027386d4a6de5d693eafcd9cd463ed3415